Method of casting titanium

ABSTRACT

A quantity of molten titanium is protected against embrittling contamination by containing all of the molten titanium within a receptacle and releasing along all titanium surfaces which are in contact with the receptacle the liquid and gaseous halide products of melting and decomposition (caused by heat of the molten titanium) of nonreactive flux consisting essentially of at least one halide which generates these protective products on exposure to temperatures of molten titanium.

United States Patent Rosenberg 51 Feb. 15,1972

[54] METHOD OF CASTING TITANIUM [72] Inventor: Robert A. Rosenberg, Norwood, Mass.

[73] Assignee: Mitron Research 8: Development Corporation, Waltham, Mass.

[22 Filed: rm.23,1968 211 Appl.No. 786,279

Related U.S. Application Data [62] Division of Ser. No. 632,127, Feb. 23, 1967, Pat. No.

[52] U.S. Cl ..l64/138, 164/72, 164/74, 164/67, 75/96, 164/41, 75/95, 75/94, 106/389 [51] Int. Cl. ..B22c l/00, B22c 3/00 [58] field of Search ..75/95, 96, 65, 94, 10, 84, 75/2, 9, 93 A, 93 B, 93 C; 266/43; 104/68;

[56] References Cited UNITED STATES PATENTS 3,417,808 12/1968 Rosenberg ,.164/68 3,035,318 5/1962 Campbell... 164/72 3,113,360 12/1963 Neff, Jr ..l64/14 2,912,729 11/1959 Webb ...l64/361 3,271,828 9/1966 Shelton et al ..164/52 Maddex ..75/84 FOREIGN PATENTS OR APPLICATIONS 968,822 4/1958 Germany 968,822 4/1958 Germany 808,843 2/ 1959 Great Britain ..75/96 OTHER PUBLlCATlONS Slag Washing of Stainless Steels Graduates from Pilot Stage Steel, 25 May 1959, pp 146, 149.

Primary Examiner-Robert D. Baldwin Assistant Examiner-V. K. Rising Attorney-David A. Rich and John W. Ericson [57] ABSTRACT A quantity of molten titanium is protected against embrittling contamination by containing all of the molten titanium within a receptacle and releasing along all titanium surfaces which are in contact with the receptacle the liquid and gaseous halide products of melting and decomposition (caused by heat of the molten titanium) of nonreactive flux consisting essentially of at least one halide which generates these protective products on exposure to temperatures of molten titanium.

l 3 Drawing Figures PAIENYEBFEB 15 Ian 3.642.056

INVENTOR ATTORNEYS METHOD OF CASTING TITANIUM This application is a division of my copending application Ser. No. 632,127, now US. Pat. No. 3,417,808, issued Dec. 24, 1968, dated Feb. 23, 1967 as a continuation of my copending application Ser. No. 507,611, filed Oct. 22, 1965, now abandoned, which latter application was a continuation of a copending application Ser. No. 325,067, filed Nov. 20, 1963, now abandoned.

The present invention relates to improvements useful in the foundry processing of titanium and, in one particular aspect, to novel and improved melting and casting practices whereby titanium masses substantially free of the common embrittling impurities are produced conveniently and at relatively low cost.

Titanium is a highly attractive metal to the engineering designer because it exhibits unusually favorable strength-toweight ratios, good ductility, and extraordinary immunity to corrosion. Certain alloys of titanium are also known to offer excellent ductility, high-endurance ratios and desirable fatigue resistance; moreover their tensile strengths, fatigue limits and hardness tend to increase at very low temperatures. Unfortunately, this metal is seriously vulnerable to contamination while in the molten state, inasmuch as it acts as a nearly universal solvent which either dissolves or is polluted by atmospheric gases and about every known refractory. Heated titanium (above about 1,100 F.) reacts so swiftly and deleteriously with these other substances that protective measures must be rigorously enforced to prevent atmospheric air and moisture from contaminating the molten material either in the furnace where it is being prepared or in the mold where it is being cast to a desired form. Evacuated or inert gas environments have been relied upon to inhibit the unwanted reactions, but, for optimum results, these environments must be rigorously controlled by skilled personnel and they tend to involve cumbersome and costly equipment. Drastic losses of ductility can result from improper protection of titanium while it is in a molten state, examples of excessive contaminations being the very low values of about 0.5 percent of oxygen and 0.25 percent of nitrogen. Hydrogen likewise has embrittling effects, while carbon adversely affects machinability and welding, and both are to be scrupulously avoided. Inert atmosphere and high-vacuum techniques require uncommon laboratory-oriented skills, equipment, and care, and they are thus not readily integrated into the usual foundry operations.

In accordance with the present teachings, the needed protections of the highly reactive molten titanium within furnace and mold can be realized without evacuation and either without substitution of an inert gas atmosphere or merely with a very simple auxiliary blanketing of inert gas. Heat-affected surfaces of the titanium are preserved free of contaminants principally by nonreactive flux which liberates a protective gas, such as a gaseous chloride, and at the same time serves as part of the container or vessel for the molten metal. In addition, the container thus formed is constructed to permit exhaust of generated gases from the flux, and, in certain practices, the exposed upper surfaces of the molten titanium are also covered with gas-liberating molten or finely divided amounts of a similar flux which provides more complete isolation from troublesome gases of the ambient atmosphere while yet being pervious and nonrigid to accommodate release of generated gases and the like.

It is one of the objects of the present invention, therefore, to provide novel and improved techniques and associated apparatus for the production of masses of high-purity titanium and its alloys in molten and cast forms.

A further object is to provide improved and economic processing of titanium sponge which yields substantially uncontaminated melts, ingots and castings of titanium.

Another object is to provide improved methods and apparatus for the furnace melting of relatively large amounts of high-purity titanium which avoids the use of complex and costly evacuation or inert-gas chambers.

Still further it is an object to provide improvements in the expensive and simple casting of ductile titanium masses by relatively unskilled personnel.

By way of a summary account of practice of this invention in one of its aspects, a quantity of low-cost titanium sponge and/or clean scrap titanium is heated to the molten state by its electrical resistance to flow of secondary currents induced by a surrounding high frequency induction coil. For this purpose, the titanium charge is disposed within an electrically nonconductive open-topped crucible which,in turn, has been coated internally with a thick substantially solid lining of a special nonreactive and substantially nonconductive flux composition. Preferably, this flux composition includes halogen compounds such as sodium chloride and potassium chloride, and the interior lining of the crucible may be formed either by applying the flux, and permitting it to solidify, or by separately casting a suitable hollow liner and inserting it into the crucible. The cavity within the lined crucible is charged with the metal, and/or titanium sponge, and appropriate excitation of the high frequency coil then results in melting of the charge within the lining. At the high temperatures of the melting titanium charge, the flux composition decomposes and liberates gaseous halides which rise through the charge and are exhausted from its upper surface, thereby shielding and isolating all exposed titanium surfaces from reactions with ambient atmospheric gaseous contaminants. Preferably, fresh amounts of the finely divided flux are added atop the charge as melting continues, and additional amounts of the charge may also be added to accumulate a large molten volume. When the melting has been accomplished, ingots and other castings are conveniently formed using a mold prepared from sand molding material which has been well mulled, or mixed, with a finely divided flux like that employed in the furnace. Pouring from crucible to mold is performed swiftly, to avoid substantial con tact with the air, and finely divided flux is added atop the cast metal to produce a shielding halide gas layer which supplants the atmospheric gases at that site.

Although the features and aspects of this invention which are considered novel are set forth in the appended claims, further details as to preferred practices of the invention, as well as the further objects and advantages thereof, may be most readily comprehended through reference to the following description taken in connection with the accompanying drawings, wherein:

FIG. 1 portrays, in cross section, an improved are furnace arrangement for producing uncontaminated melts of titanium;

FIG. 2 depicts induction furnace melting of titanium having low impurity content, the equipment being shown in section; and

FIG. 3 represents the operation of sand-casting high-quality ductile titanium in accordance with the present teachings, a portion of the illustrated special sand-type mold being broken away to expose its interior.

The metal-melting furnace arrangement characterized in the FIG. 1 illustration is of the direct-arc type, wherein an electric arc 4 is drawn directly between an electrode 5 and a titanium charge 6 beneath it within a crucible 7. For purposes of preparing highly ductile titanium, it is important that those materials which are embrittling contaminants of titanium be avoided, and the electrode 4 is thus preferably formed of a metal such as tungsten or, in the case of consumable electrodes, of titanium sponge which has been prepared in a suitable length for feeding. The crucible 7 is preferably made of copper, which possesses good thermal conductivity, and this is of a force-cooled jacketed construction involving the circulation of a coolant such as water through the couplings 7, and 7,, and in heat-exchange relationship with the inner walls of the jacketing. Electrode 5 is also preferably cooled, particularly in those instances when a nonconsurnable electrode is employed, and a water-cooled electrode holder 8 is shown for this purpose. An electrical power source (not shown) of a conventional form provides excitation of the electrode, as designated by the coupling 9,, and of the metal charge and molten pool within the crucible, the latter by way of the conductive crucible and coupling 9,,. Are 4 is responsible for generation of heat which rapidly melts the charge and forms a pool 6,, of titanium which may be tilt-poured into a mold via the crucible spout; in another known type of furnace construction, the molten metal may instead be tapped from below, through a drain at or near the bottom of the furnace.

Before melting is commenced, those interior surfaces of the copper crucible which are expected to come in contact with molten titanium are heavily lined with a special nonreactive flux composition. Thick lining in FIG. 1 is illustrative, and this may for example be formed either as a separately cast solid part or as a liquid material applied to the cool interior surfaces and allowed to solidify there until the desired thicknesses of flux composition have been attained. The materials used in the flux compositions are of pronounced importance, inasmuch as they should not so react with the solid or molten titanium as to embrittle it and, moreover, they should release nonreactive gas which can supplant or flush out any potentially contaminating gases in the air near surfaces of the titanium charge. These requirements are satisfied by certain halide salts which do not react appreciably with titanium up to the highest temperatures expected in the melting operation. Relatively plentiful and inexpensive salts such as potassium and sodium chlorides (KCl and NaCl) may be used, with typical percentages running from about 55-65 percent NaCl and 35-45 percent KCl, by weight. In another example, stannous chloride, SnCI may also be added advantageously, preferably in small amounts, such as about 1-2 percent, by weight. This chloride starts melting at a relatively low temperature (of the order of 500 F., and more specifically, about 475 F.; the boiling point being about l,l 53 F.) and it thus affords the desired type of shielding protection before the titanium charge has actually begun to melt but while it is hot enough to be highly susceptible to contamination. The other chlorides melt and are principally effective at much higher temperatures; KCl melts at about l,040 K. and vaporizes at about l,680 K. Thermodynamic calculations of the free energies of reaction between KCl, for example, and titanium to form the possible chlorides of titanium (Ti Cl TiCl and TiCl.,) provide evidence of the relative inertness of the compound for the present purposes (i.e., the free energy of forma tion of TiCl, in reaction with KCl is but about 112 kilogram calories per mole, at 2,500 K.; of TiCl but about 69 Kg.cal/mole; and of TiCl but about 40 Kg.cal/mole). Magnesium chloride (MgCI is found to be a very desirable material for these purposes also. Similar conditions of nonreactivity with titanium can be found in the case of the other halides of interest, the characterizing free energy of reaction, Fr, at 2,500 K. being as follows (in Kg.cal/mole) in selected instances:

in addition to the chlorides and fluorides, bromides and iodides are suitable agents, the latter having the greater atomic weights and, thus, decreased reactivity. The hygroscopic nature of the halides may in some cases make it desirable that the heated or molten titanium be protected from the contaminating effects of the water from that source. For this purpose, a small quantity of a hydroxide-forming oxide, such as Na 0, Li O or K, O is included in the flux composition; up to 5 percent of this material by weight of the entire flux composition serves this need. These oxides react readily with any moisture which may be present, especiallyat elevated temperatures, to form hydroxides (such as Na OH, Li OH and K OH) which do not decompose and which therefore provide the desired protective isolation from such moisture. in addition to the halides and hydroxide-forming oxides, the nonreactive flux composition may include finely divided titanium sponge. To the extent that such sponge may melt, it introduces no significant impurities and merely becomes part of the molten titanium pool in the furnace, and, to the extent that it does not, it serves as an essentially nonreactive filler and binder for the halide-containing flux composition.

As the molten titanium pool 6, is developed in response to the intense heating which attends the generation of the are 4, additional mounts of titanium sponge and/or clean titanium scrap may be introduced, together with quantities of the nonreactive flux material, the latter preferably in a finely divided form. Arrow 11 in FIG. 1 suggests this, and indicates the path through which both materials may be dumped into the opentopped crucible 7 as needed. Titanium sponge which may be added to the crucible in this manner may also advantageously by in a finely divided state. Enough of the halide fluxing agent must be present to insure that the top surfaces of the molten titanium pool 6,, do not become contaminated by the troublesome gases from the ambient atmosphere; preferably, this agent forms a substantially full protective layer 12 atop the molten titanium, some of which layer may comprise melted flux while the balance includes partially melted or unmelted flux which has not decomposed and which may be of the finely divided form. Depending upon the temperatures which are experienced by the flux composition, it will produce a nonreactive liquid or gaseous film, or both, atop the molten titanium and will thereby effectively screen out possible contaminants which could seriously deteriorate the resulting ductility characteristics of the cast product. As the melting progresses, layer 12 may at least in part be formed by amounts of the lining 10 which may have melted under furnace heat and which have surfaced through the molten titanium pool. However, this lining does not become completely melted away or decomposed, because of the inhibiting cooling effects of the circulated coolant in the water-cooled copper crucible 7, and because of the lining thickness selected. Preferably, a substantially continuous cover (skull" or skin") of molten flux is maintained over the exposed top surface of the, molten titanium throughout the melting process, although this way in some instances require that only relatively small quantities of the finely divided or hot liquid flux material be added during the melting operation, the balance being supplied automatically upon gradual melting of the flux liner 10. Additional protection, if desired, can also be developed cooperatively by blanketing the top surfaces of the charge with inert gas, such as argon. This may be accomplished by at least partially covering the crucible and feeding the inert gas into the space below the cover. Finely divided titanium, preferably in the form of titanium sponge, may be included in admixture with the flux material of the liner, to vary the thermal conductivity characteristics of that liner without introducing any impurity which would adversely affect the resultant melt. Thicknesses of the liner walls are selected, empirically, for example, to insure that it, the liner, is not completely decomposed at any position about the molten pool during any one melting operation. A full charge of molten titanium may be built up within a cruelble, starting with a large unmelted mass therein, or by adding more and more titanium to the crucible as smaller quantities are melted. Alternatively, molten amounts may be tapped or poured off soon after they appear.

The FIG. 2 furnace arrangement for induction melting of titanium in accordance with these teachings includes the usual conductive induction coils 13 in surrounding relationship to a ceramic crucible l4 and the packed refractory cement enclosure 15. Alternating current excitation of a suitable frequency is applied to the coil via leads l6 and 17, and, preferably, a coolant such as water is forced through the hollow coils 13 to suppress their thermal deterioration. Blocks 18 of refractory material complete a conventional structure holding the coil and crucible arrangement within a sturdy metal framework 19 which preserves mechanical integrity enabling the assembly to be tilted in a stand (not shown) for pouring. Within crucible 14 there may be nested a solid lining 20 of the special flux composition, of formulation as disclosed hereinabove, except that preferably there is no significant amount of titanium sponge within the flux composition. This lining serves to protect the molten titanium from contaminants, melting and decomposing to yield a protective liquid and gaseous blanketing atop the melt 21. in addition, this blanketing 22, is preferably augmented by the addition of either hot liquid or finely divided flux deposited from above, as needed, and as designated by the arrow 23. Alternatively, all of the protective flux composition may be applied to the top of the charge or melt, in the liquid or finely divided form. The titanium charge being melted (titanium sponge, scrap, or the like) offers resistance to the flow of secondary electrical currents induced in it by electromagnetic induction, thereby developing intense heat which causes the desired melting, in a manner well known in the art. This process can be carried on without using any protective enclosures. However, in some instances it can also be helpful to maintain an artificially created protective gaseous atmosphere, such as one of argon, either with or without adding the flux atop the charge. In the latter event, a convenient technique involves introducing the inert gas below a simple cover (not illustrated) which has openings permitting the gases to escape while at the same time preserving a blanket of gas below it to screen out the ambient atmosphere.

The extraordinarily troublesome contamination problems which are encountered in the furnace-melting of titanium are also present in connection with the casting which follows, and it is important that these be minimized if the full benefits of the improved melting practices are to be realized. Gases from the ambient air can be of insignificant contaminating effect if the pouring or drainage from furnaces like those of FIGS. 1 and 2 is accomplished swiftly. However, these gases can readily cause contamination as the molten titanium is taking shape and solidifying within the mold. Oxygen in the sand-molding material of the parts of a mold is of serious consequence. To overcome these difficulties, one or both of two practices should be adopted. One of these involves the treatment (as by mixing or mulling") of the sand material used in the mold with a quantity of the aforementioned flux composition, the latter in one case being in a granular form for uniform admixture with and distribution throughout the mold sand material and, in another case, being in fluid form for brushing, spraying or the like. The other practice involves the application of a quantity of the special flux composition to the top surfaces of the metal in the mold, either in liquid or finely divided form. In FIG. 3, for example, the mold box 24 contains a simple type of coreless mold 25 having both the sand mold material and flux granules in an admixture which includes sand material in quantity needed to develop the customary integrity of mold shape and which includes the granular flux in quantity sufficient to protect the cast titanium from contaminants by melting and releasing the protective gas when the molten titanium is introduced and solidifies. The heat of the titanium poured into the mold from a crucible, 7", causes the desired flux decompositions to occur, without more being done. A discharge tube 26 from a suitable reservoir of the flux composition (not shown) releases the melted and/or finely divided flux composition 27 downwardly into the sprue cup 28 and onto the top of the titanium 29 after it is poured from the crucible. Preferably, the same flux composition is also introduced into any vents or risers, to protect the molten titanium mass 30 there also. Importantly, the melted and/or finely divided flux composition permits gases and other surfacing matter to escape or float at the top, while at the same time maintaining a substantially full protective cover until the molded ingot or other molded product has become fully solidified and substantially immune to embrittling contamination. Another useful technique involves the coating of the exposed surfaces of mold parts, such as the interior surfaces of the mold 25, with a liquid form of the flux by spraying, brushing, or the like. In certain instances, this may be done in addition to mixing the mold material with the flux in powdered or granular form, to further promote the protective effects which can be obtained. The mold material (or sand," as it is known in the art) which may be used with the flux in making the usual mold parts, including subsections and cores, include the customary refractory materials such as silica, alumina, zirconia, zircon, graphite, and others such as are listed by Taylor, Flemings and Wulff on page 40 of their text Foundry Engineering, published by John Wiley & Sons, Inc., N.Y., 1959.

Certain of the techniques, structures and formulations selected for discussion here for purposes of adequate disclosure are intended to be representative, rather than limiting, in character. The furnace and mold constructions may obviously be of forms different from those illustrated, while yet lending themselves to exploitations of the protective concepts. The halide fluxing agents may in appropriate instances include fine titanium sponge, such as about 2 percent by weight, or may be made more fluid or pastelike by the addition of a liquid binder such as titanium tetrachloride and silicon tetrachloride, which does not release harmful hydrogen or carbon. Additive oxides such as Na O, Li O or K 0, which react with any water present in the hygroscopic halides to form hydroxides which will not decompose during the melting or casting, may also be introduced in typical amounts up to about 5 percent by weight. The halide cover for the titanium, while being fluid or porous to permit escape of gases and the like, also advantageously restrains the escape of heat in the case of furnaces and molded products which should be cooled slowly.

Therefore, although specific practices have been described and preferred embodiments and materials have been discussed, it should be understood that various changes, modifications, additions and substitutions may be effected by those skilled in the art without departure from these teachings, and it is aimed in the appended claims to embrace all such variations as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the casting of titanium, the process of maintaining a quantity of titanium including molten titanium protected against contamination, which comprises:

pouring molten titanium into a mold and releasing a nonreactive flux between the surfaces of said molten titanium and said mold, said flux consisting essentially of at least one halide that melts at or below the temperatures of molten titanium,

said halide including an amount of stannous chloride sufficient to produce an effective protective coating of the surfaces of said titanium with liquid and gaseous stannous chloride upon contact with said titanium as it solidifies and cools below the melting point,

wherein said mold includes said flux mixed with other refractory materials. 

